This invention relates to audio circuits and, more particularly, to audio circuits used for amplification and interface purposes.
Various audio amplifier circuits are available, but they could be improved. For example, such circuits conventionally use transformers to convert the input signals to the desired output levels. Such transformers add undesirable cost and weight to the circuit. In addition, conventional circuits are not particularly well suited for driving multiple parallel outputs. For example, at a press conference a single microphone may be best positioned to receive the speaker's voice, but a multitude of persons may want a feed from that microphone. Conventional circuits are not well suited to provide such multiple feeds without undesirable distortion and loss of signal strength. Moreover, the frequency response of conventional audio amplifier circuits, while adequate, could also be improved.
Problems also arise when currently available transformer based circuits are used with charged microphones. These microphones require a source of current, but that current is typically not available from the transformer circuit because of the inability of such circuits to pass dc power. A separate box must generally be used to solve this problem, with the attendant inconvenience and cost.
Audio amplifier circuits are used with various types of equipment, such as microphones, musical instruments, etc., which have different impedances. It would be desirable to have a single audio amplifier circuit which could readily accommodate such diverse pieces of equipment, but conventional circuits are not well-suited for that task. Prior art equipment typically has a preset input impedance, or at most a choice of two preset input impedances. These impedances remain unchanged during use of the equipment and the resulting impedance mismatches adversely affect the signal characteristics. The prior art equipment provides for variation in gain to somewhat compensate, but even with variation in gain the underlying problem--the impedance mismatch--remains.
In addition, conventional audio circuits are not always well suited to provide the level shifting which is required with modem communications systems. For example, a radio call-in program requires feeding a telephone signal to a radio station console, which operates at a different level. As a result of inadequate conversion of these levels, the telephone caller's voice when broadcast over the air invariably sounds different than the studio voices. In addition, an undesirable hum may arise in these circumstances. Similarly, remote television feeds suffer from some loss of fidelity as well. Such loss of fidelity can arise not only from inadequate level shifting, but also from variations in ground reference, which are practically unavoidable with conventional equipment.
Signal degradation also occurs in existing equipment when the signal is transmitted over a high capacitance cable. Currently available amplification circuits are generally unable to perform acceptably in these circumstances.
Similar problems arise in matching the signal levels of various audio inputs (such as microphones or musical instruments) to the requirements of recording equipment. Due to such differences, a console (which is not inexpensive) is typically required between the various input devices and a recording device such as an Alesis sound recording deck. It would be a great advantage to have a relatively simple, inexpensive interface circuit which would allow any of a number of different audio input devices to be used directly with a single piece of recording equipment.